The present invention relates generally to line severing devices. More particularly, the present invention concerns pyrotechnically actuated explosive line cutters.
Explosive line cutters have been known for many years. These devices are particularly useful, for example, in the staged deployment of parachutes. Where a drogue chute is used to deploy a main chute, in some instances the drogue chute is subsequently jettisoned and in other instances, the main parachute is deployed in stages, which stages are controlled by reefing lines. A line cutter is a useful device to permit reefing lines to be severed sequentially during staged deployment and also to effect disengagement between a drogue chute and a main chute, where necessary. In other applications, line cutting devices can be used to release a load from an aerial balloon to permit controlled descent of that load by parachute. The foregoing constitute but a few of the examples of the usefulness of explosive line cutting devices.
It will be readily appreciated that a considerable time period may pass between the time an explosive line cutting device is manufactured and the time when the line cutting device is actuated. Thus, it is particularly important that the line cutting devices be highly reliable.
Moreover, it will be seen that where the line cutting device is exposed to the atmosphere it must also operate reliably through a wide range of altitudes, pressures, temperatures and humidity conditions. To the extent that these atmospheric variations can affect a pyrotechnically actuated device, steps must be taken to ameliorate or eliminate any adverse effects resulting from variations in those parameters. In addition, since a line cutting device is often associated with the deployment of a parachute, and since the movements of a parachute during deployment cannot be predicted with a high level of reliability, it will also be appreciated that a line cutting device must be actuated with the line cutting device in a wide variety of positional attitudes.
Parachute line cutting devices are also known in the prior art. A typical line cutting device includes an anvil and a knife which is movable toward the anvil but spaced therefrom by a distance which permits the line to be severed to pass therebetween. The blade is typically propelled by a conventional explosive actuator assembly which may include a primer, a delay charge and an explosive charge. Typically, such line cutters are actuated by a spring-loaded firing pin which is held in a cocked position by a generally inflexible rod. When the rod is removed from the cutter assembly, the compressed spring quickly plunges the firing pin into the primer mix of the actuator assembly. Such a known device is illustrated for example by U.S. Pat. No. 3,452,631 issued to Brizzolara on July 1, 1969 and by U.S. Pat. No. 3,419,960 issued to Hanson on Jan. 7, 1969.
Another type of line cutting device relies upon an electrical circuit to actuate the explosive charge which, in turn, forces a movable knife into engagement with the line to be severed. Such a line cutting device is illustrated, for example, in U.S. Pat. No. 3,991,649 issued to Patrichi on Nov. 16, 1976 and in U.S. Pat. No. 2,924,147 issued to Bohl et al on Feb. 9, 1960.
Different types of devices have also been proposed for explosively destroying part of a line rather than cutting the line. For example, it has been proposed to use an explosive charge which is embedded in a braided rope or cable and connected to an igniting device by a slow match compound. The igniting device may, for example, include a fulminating compound surrounding a barbed wire which compound is fired when the barbed wire is withdrawn. Such a destructible link device is shown, for example, in U.S. Pat. No. 2,768,802 issued to Dejean on Oct. 30, 1956.
Where a parachute or similar article has been deployed and is to open in stages, there is typically no available source of electrical power. Thus, those parachute line cutting devices known in the prior art which require an electrical impulse for actuation are generally unsuitable for staged deployment of remotely operated articles such as parachutes.
Moreover, a reefing line cutting device which uses a rigid rod to release a precocked triggering mechanism cannot be actuated unless the lanyard rod is pulled in a direction almost coaxial with the line cutting device itself. Therefore, in applications where the positional attitude of the line cutting device cannot be predicted with certainty, a precocked line cutting device having a rod-type lanyard is also unsuitable.
There are serious disadvantages associated with those line cutting devices that use either precocked or non-precocked spring. Prococked springs have stored energy which is released when a lanyard is pulled whereas non-precocked springs energize a spring when the lanyard is pulled. The disadvantages of these springs include the need for additional components to obtain the necessary functions of the line cutting device. Moreover, since the line cutting device normally has a two year shelf life, a precocked spring can develop a permanent set during this period of time which reduces the releasable stored energy of the spring. Whether the spring is precocked or non-precocked, undetected fractures in the spring can render the line cutting device inoperative.
In addition there is an operational disadvantage to spring actuated line cutting devices. The line cutting device is often actuated in a dynamic force environment such as a rapidly decelerating falling body. Accordingly, when the operating lanyard is removed, deceleration forces often oppose the resilient movement of the spring thereby impeding its function.
Another common disadvantage of the known line cutting devices is that aspect of their construction which permits gases created during the explosion to expand throughout substantially the entire internal cavity. Such a construction requires a larger explosive charge than is required when the gases are constrained to remain in proximity to the movable blade.
Furthermore, the known pyrotechnically actuated line cutting devices which are provided with a delay compound experience variations in delay time between +25% and -15%. Such variations depend on the ambient temperature and are unacceptable in critical deployment situations.
Accordingly, it will be apparent to those skilled in the art that the need continues to exist for an improved line cutting device which is relatively insensitive to positional attitude, reduces delay time variations, free of electrical actuation, concentrates explosive force at the movable knife and is environmentally sealed. Therefore, it is the object of the present invention to provide a line cutting device which exhibits these desirable features.